Organic fuels can be used to generate electrical power by converting energy released from electro-chemical reactions of the fuels. Organic fuels, such as methanol, are renewable. Typical products from the electro-chemical reactions are mostly carbon dioxide and water. These products are environmentally safe. Therefore, organic fuel cells are considered as an alternative energy source to non-renewable fossil fuels for many applications. In addition, use of fuel cells can eliminate many adverse environmental consequences associated with burning of fossil fuels, for example, air pollution caused by exhaust from gasoline-powered internal combustion engines.
Direct liquid-feed oxidation fuel cells are of particular interest due to a number of advantages over other fuel cell configurations. For example, the organic fuel is directly fed in to the fuel cell. This eliminates the necessity of having a chemical pre-processing stage. Also, bulky accessories for vaporization and humidification in gas-feed fuel cells are eliminated. Thus, direct liquid-feed cells generally have simple cell construction and are suitable for many applications requiring portable power supply.
Conventional direct liquid-feed cells usually use a liquid mixture of an organic fuel and an acid/alkali electrolyte liquid, which is circulated past the anode of the fuel cell. Problems associated with such a conventional direct liquid-feed cell are well recognized in the art. For example, corrosion of cell components caused by the acid/alkali electrolyte places significant constraints on the materials that can be used for the cell; fuel catalysts often exhibit poor activity due to adsorption of anions created by the acid electrolyte; and the use of sulfuric acid electrolyte in multi-cell stacks can result in parasitic shunt currents. As a result, the performance of the conventional cells is limited to about less than 0.3 volt in output voltage and less than about 30 mA/cm.sup.2 in output current. In addition, a number of safety issues arise with the use of acidic and alkaline solutions.
NASA's Jet Propulsion Laboratory (JPL) developed an improved direct liquid-feed cell using a solid-state membrane electrolyte. One of the advantages of the JPL fuel cell is the elimination of the liquid acidic and alkaline electrolyte by the membrane electrolyte. This solves many problems in the conventional fuel cells. A detailed description of JPL's fuel cell can be found, for example, in U.S. Pat. No. 5,599,638 and in U.S. patent application Ser. No. 08/569,452, filed on Dec. 8, 1995, now U.S. Pat. No. 5,773,162, both of which are incorporated herein by reference.
FIG. 1 shows a typical structure 100 of a JPL fuel cell with a membrane electrolyte 110 enclosed in housing 102. The electrolyte membrane 110 is operable to conduct protons and exchange cations. An anode 120 is formed on a first surface of the membrane 110 with a first catalyst for electro-oxidation and a cathode 130 is formed on a second surface thereof opposing the first surface with a second catalyst for electro-reduction. An electrical load 140 is connected to the anode 120 and cathode 130 for electrical power output.
The membrane 110 divides the fuel cell 100 into a first section 122 on the side of the anode 120 and a second section 132 on the side of the cathode 130. A feeding port 124 in the first section 122 is connected to a fuel feed system 126. A gas outlet 127 is deployed in the first section 122 to release gas therein and a liquid outlet 128 leads to a fuel re-circulation system 129 to recycle the fuel back to the fuel feed system 126. In the second section 132 of the cell 100, an air or oxygen supply 136 (e.g., an air compressor) supplies oxygen to the cathode 130 through a gas feed port 134. Water and used air/oxygen are expelled from the cell through an output port 138.
In operation, a mixture of an organic fuel (e.g., methanol) and water is fed into the first section 122 of the cell 100 while oxygen gas is fed into the second section 132. Electrochemical reactions happen simultaneously at both the anode 120 and the cathode 130, thus generating electrical power. For example, when methanol is used as the fuel, the electro-oxidation of methanol at the anode 120 can be represented by EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ 6e.sup.-,
and the electro-reduction of oxygen at the cathode 130 can be represented by EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O.
Thus, the protons generated at the anode 120 traverse the membrane 110 to the cathode 130 and are consumed by the reduction reaction therein while the electrons generated at anode 120 migrate to the cathode 130 through the electrical load 140. This generates an electrical current from the cathode 130 to the anode 120. The overall cell reaction is: EQU 2CH.sub.3 OH+3O.sub.2 .fwdarw.2CO.sub.2 +4H.sub.2 O + Electrical Energy.
The inventors recognized the advantages and potential of the JPL's membrane fuel cell. Importantly, the inventors have discovered a number of new materials for various components and processing methods that can be used to improve the performance of this type of fuel cells.
One aspect of the present invention describes new material compositions for catalysts with improved efficiency and methods for forming catalyst layers on the membrane electrolyte including transfer of catalyst decals and deposition of catalyst materials onto a backing layer with minimized catalyst permeation.
Another aspect directs to improve catalyst efficiency and reactivity by increasing the surface area thereof.
Yet another aspect is to increase the reactivity of a catalyst by changing the electronic properties of a catalyst layer.
Still another aspect of the invention is construction and processing of the electrolyte membrane to improve coating, bonding, and to reduce fuel crossover.